This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The structural maintenance of chromosomes (SMC) proteins, which are evolutionarily conserved from prokaryotes to eukaryotes, are essential for higher order chromosome organizations and functions. The Smc5/6 holocomplex, formed by a heterodimer of Smc5 and Smc6 in association with six non-SMC components is localized on repetitive rDNA and telomere, and is important for DNA replication and double stranded DNA damage repair through homologous recombination. In the Smc5/6 holocomplex, the non-SMC component Mms21, a SUMO E3 ligase, is important for Smc5/6-mediated DNA damage repair, localized on the coiled-coil region of Smc5. The molecular mechanism of the function is unclear. In addition, limited structural information is available on Smc5/6. To fill this gap, we are determining the structure of the coiled-coil region of Smc5 with the Mms21 complex.